Behavior of iron-based oxygen carriers at deep reduction states

Doctoral thesis, 2024

Oxygen carriers have an important role as bed materials in both common circulating fluidized bed combustion unit (also known as oxygen-carrier-aided combustion, OCAC) and in various chemical looping processes. Contrary to conventional bed materials, e.g., sand, oxygen carriers are capable of transferring both oxygen and heat. This makes it possible to produce nitrogen-free product gas streams (in the case of chemical looping processes) and achieve a higher fuel conversion. Having been studied for almost three decades, various oxygen carriers show their own pros and cons depending on the processes for which they are intended to be used. Most of the published studies before this doctoral thesis focused on the reactivity and utilization of oxygen carriers in chemical looping combustion (CLC), where a complete fuel conversion is always desired. Nevertheless, this is not the case in, for example, chemical looping gasification, reforming, and water splitting, where only partial fuel oxidation is necessary, and the produced flue gases are the desired products. In such processes, the oxygen carriers can be exposed to a higher reduction degree than it would be in CLC or OCAC. This warrants further investigations into the deep reduction states of relevant oxygen carriers, which are expected to encounter inevitable performance issues under such a harsh environment.

In this thesis, some aspects related to the physical performance and properties of various iron-based oxygen carriers in the occurrence of deep reduction states are examined and presented. The first part of the thesis focuses on the fluidization performance, attrition resistance, and particle size and shape analysis. This part is important mainly for assessing material stability. Iron-based oxygen carriers typically tend to encounter bed defludization at a high degree of reduction. The outward migration of iron into the particle surface, which typically creates a FeO/Fe layer, likely causes defludization. Furthermore, the oxidation state of oxygen carriers does affect the attrition resistance of iron oxygen carriers to varying extents. The results indicate that the presence of Fe-Ti and Fe-Si combinations contribute to a generally stable and low attrition rate, while an Fe-Ca system exhibits a decreasing attrition rate. In addition, the influence of exposure to redox cycles and oxidation degree on the size and shape of oxygen carrier particles seems to be minimal. The oxygen carrier particles generally have a high sphericity but are slightly elongated.

Reactivity and fuel conversion are the other focuses of this thesis. These have main implications for engineering design but also for material screening. The apparent kinetic study of oxygen carrier performed in this thesis demonstrates that the changing grain size (CGS) model is applicable to predict the reactivity of three iron oxygen carriers in the presence of CO, H2, and CH4. This applies even at lower oxidation degrees (3 – 5 wt.% reduction), where the reactivity of oxygen carriers has generally decreased. Finally, the gasification rate of pine forest residue char remains at similar levels when using either ilmenite or iron sand as the oxygen carrier.